In PyJWT 1.5.0 and below the
invalid_strings check in
HMACAlgorithm.prepare_key does not account for all PEM encoded public keys. Specifically, the PKCS1 PEM encoded format would be allowed because it is prefaced with the string
-----BEGIN RSA PUBLIC KEY----- which is not accounted for. This enables symmetric/asymmetric key confusion attacks against users using the PKCS1 PEM encoded public keys, which would allow an attacker to craft JWTs from scratch.